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Part 3 — no species lives alone: how populations of different species affect each other.
In this lesson you will
| Species A | Species B | Interaction |
|---|---|---|
| Mutualism | ||
| − | − | Competition |
| − | Predation | |
| − | Parasitism | |
| 0 | Commensalism | |
| − | 0 | Amensalism |
Predation is nature’s way of transferring energy fixed by plants to higher trophic levels. Predators also keep prey populations under control and help maintain species diversity by reducing competition among prey.
| Case | What happened |
|---|---|
| Prickly pear cactus in Australia (early 1930s) | Spread out of control until a cactus-feeding moth from its natural habitat was introduced |
| Starfish Pisaster removed from rocky shores of the American Pacific coast | More than 10 invertebrate species became extinct within a year because of competition |
Prey species have evolved defences. Nearly 25 percent of all insects are plant eaters (phytophagous), and plants, which cannot run, rely on morphological and chemical defences.
| Defence | Example |
|---|---|
| Camouflage (cryptic colouring) | Some insects and frogs |
| Distasteful chemicals | Monarch butterfly: chemical acquired by its caterpillar from a poisonous weed |
| Thorns | Acacia, cactus |
| Poisons | Calotropis: cardiac glycosides, so cattle and goats avoid it |
| Chemicals used by humans | Nicotine, caffeine, quinine, strychnine, opium |
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